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Category: algorithms Component type: function


template <class InputIterator1, class InputIterator2>
int lexicographical_compare_3way(InputIterator1 first1, InputIterator1 last1,
                                 InputIterator2 first2, InputIterator2 last2);


Lexicographical_compare_3way is essentially a generalization of the function strcmp from the standard C library: it returns a negative number if the range [first1, last1) is lexicographically less than the range [first2, last2), a positive number if [first2, last2) is lexicographically less than [first1, last1), and zero if neither range is lexicographically less than the other. [1]

As with lexicographical_compare, lexicographical comparison means "dictionary" (element-by-element) ordering. That is, lexicographical_compare_3way returns a negative number if *first1 is less than *first2, and a positive number if *first1 is greater than *first2. If the two first elements are equivalent [2] then lexicographical_compare_3way compares the two second elements, and so on. Lexicographical_compare_3way returns 0 only if the two ranges [first1, last1) and [first2, last2) have the same length and if every element in the first range is equivalent to its corresponding element in the second.


Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h. This function is an SGI extension; it is not part of the C++ standard.

Requirements on types

  • InputIterator1 is a model of InputIterator.
  • InputIterator2 is a model of InputIterator.
  • InputIterator1's value type is a model of LessThanComparable.
  • InputIterator2's value type is a model of LessThanComparable.
  • If v1 is an object of InputIterator1's value type and v2 is an object of InputIterator2's value type, then both v1 < v2 and v2 < v1 are defined.
  • Operator< is a strict weak ordering, as defined in the LessThanComparable requirements.


  • [first1, last1) is a valid range.
  • [first2, last2) is a valid range.


Linear. At most 2 * min(last1 - first1, last2 - first2) comparisons.


int main()
  int A1[] = {3, 1, 4, 2, 8, 5, 7};
  int A2[] = {3, 1, 4, 1, 5, 9, 3};
  int A3[] = {1, 2, 3, 4};
  int A4[] = {1, 2, 3, 4, 5};

  const int N1 = sizeof(A1) / sizeof(int);
  const int N2 = sizeof(A2) / sizeof(int);
  const int N3 = sizeof(A3) / sizeof(int);
  const int N4 = sizeof(A4) / sizeof(int);

  int C12 = lexicographical_compare_3way(A1, A1 + N1, A2, A2 + N2);
  int C34 = lexicographical_compare_3way(A3, A3 + N3, A4, A4 + N4);

  cout << "A1[] and A2[]: " << C12 << endl;
  cout << "A3[] and A4[]: " << C34 << endl;


[1] Lexicographical_compare_3way is almost, but not quite, redundant: the call lexicographical_compare_3way(f1,l1, f2,l2) could be written as lexicographical_compare(f1,l1, f2,l2) ? -1 : (lexicographical_compare(f2,l2, f1,l1) ? 1 : 0). The single call to lexicographical_compare_3way, however, is much faster than the two calls to lexicographical_compare.

[2] "Equivalent", not "equal", because two equivalent elements (that is, two elements with the property that neither one is less than the other) are not necessarily equal. Operator< is required to induce a strict weak ordering, not necessarily a total ordering. See the LessThanComparable requirements for a discussion.

See also

lexicographical_compare, equal, mismatch, search, LessThanComparable, StrictWeakOrdering, sort

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